Browsing the PSN is like the same as browsing the internet on your PC.
Playing games on the PSN however depends on which games you are playing online. Most games is about 20mb/hour. COD4 I heard is about 50mb/hour.
Some of the game demos on PSN will take a fair chunk of data allowance. You will know those games when you start downloading and it would show you how long/many gigs left to go.
PS Home can be quite heavy on data allowance as well. Vidzone (PS3 music app) takes roughly (maybe abit more) than Youtube.
If you are connecting to PSN, Uncharted 2 (once you have done ALL the updates (think it was around 150mb)), is smooth when you are in the game, however game-matching can take a couple of minutes.
7G/14G data allowance would be OK but if you are a heavy downloader, you may want to reconsider upgrading the plan to the next level.